Suzie S. Rigby

Do aromatic transition states lower barriers to silatropic shifts? A synthetic, NMR spectroscopic, and computational study

Abstract In a previous calculational study at the semi-empirical unrestricted Hartree-Fock (UHF) level of theory, we predicted that strategically incorporating an aromatic ring onto indenyltrimethylsilane would lower the barrier for [1,5]-silatropic shifts around the five-membered ring through retention of aromatic character in the transition state and in the intermediate iso -indene. We now report the synthesis and dynamic behavior of the tricyclic system angular trimethylsilylbenzindene, and also the tetracycle trimethylsilylcyclopenta[ l ]phenanthrene. Incorporation of one aromatic ring onto the indenyl ligand does lower the ΔG ≠ value for [1,5]-silatropic shifts to 21.9±0.5 kcal mol −1 , compared to 24 kcal mol −1 for the indenyl system. Addition of a second aromatic ring further lowers the barrier to 17.6±0.2 kcal mol −1 , good agreement with calculations. The intermediate iso -indenes for both systems have been trapped as their Diels-Alder adducts with tetracyanoethylene.

The barriers to trimethylsilyl migrations in indenes and benzindenes: Silatropic shifts via aromatic transition states

Abstract The barriers to migration of a trimethylsilyl group around the five-membered ring of indene, and of linear and angular benzindenes, have been calculated at the unrestricted Hartree-Fock (UHF) level of theory by using AMPAC and MOPAC. It is found that the retention of aromatic character in the transition state and in the intermediate isoindene lowers the barrier for the silatropic shifts. A preliminary experimental study on a trimethysilyl derivative of angular benzindene reveals that Me 3 Si migration is indeed a facile process. However, in the corresponding bis-trimethylsilyl systems, rearrangements which require the intermediacy of geminal Me 3 Si groups have rather high barriers.

Ferrocenes derived from cyclopenta[l]phenanthrene: dibenzindene–metal complexes that resist haptotropic shifts

Abstract The complexes (η5-cyclopenta[l]phenanthrenyl)MLn, where MLn=Fe(C5H5), Fe(C17H11), Mn(CO)3 or Rh(C2H4)2 show no propensity to undergo η5 to η6 haptotropic shifts upon protonation, nor to undergo ready replacement of a carbonyl or ethylene ligand by a phosphine. Thus, these systems mimic the behavior of cyclopentadienyl rather than indenyl rings. Molecular orbital calculations on (C17H11)Fe(C5H5) indicate that migration of an organometallic fragment from a peripheral arene into the five-membered ring is thermodynamically favorable, but that a least-motion pathway passing through the central six-membered ring is strongly disfavored. Attempted synthesis of (η6-cyclopenta[l]phenanthrene)Cr(CO)3 gave instead the corresponding dihydro complex, (C17H13)Cr(CO)3, and the Diels–Alder dimer of cyclopenta[l]phenanthrene with its own isoindene.